The first day after Apple TV began shipping a bunch of sharp-as-a-tack coder types hacked Apple's new set top box to shreds:
Non-Apple TV owners can enjoy the out of box experience by viewing the opening video which one crafty person ripped from the hard drive and posted in all of it's 720p glory. You can also download the Quartz Composer Screen Saver and the Now Playing Screen. And if you're truly hard-core you can download the entire Apple TV OS, and (conceivably) install it on another Mac.
But this is just scratching the Apple TV surface.
If you really want to hack it to the next level you can play Xvid movies on Apple TV, but it ain't pretty and involves removing the HDD (covered above) and un-breaking SSH (using Perian and DropBear) so you can access the Apple TV remotely.
Keep up with even more Apple TV hacks at AppleTVHacks.net.